1. Field of the Invention
The present invention relates generally to a magnetic disk drive, and particularly to a disk drive adopting a perpendicular magnetic recording system.
2. Description of the Related Art
In recent years, perpendicular magnetic recording has been noticed as a technique to overcome the limits of recording density of longitudinal magnetic recording in the field of magnetic disk drives represented by hard disk drives.
In perpendicular magnetic recording, when digital data (0/1) is recorded on a disk medium, a magnetizing area is formed for the data in a direction perpendicular (depth) to the disk medium. A read signal read by a read head from the disk medium has a rectangular waveform as a result of changing its amplitude at the magnetizing direction changing point and adapting to the changed magnetizing direction.
Further, recent disk drives adopt a magnetic head in which the read head and write head are separately mounted on a slider. The write head is usually a single-pole type head suitable for perpendicular magnetic recording. The read head usually employs a GMR (Giant Magnetoresistive) element or a spin valve type MR element.
In perpendicular magnetic recording, the read signal read by the read head from the disk medium is changed in its amplitude at the point of changing the disk magnetizing direction and adapted to the changed magnetizing direction, and resultantly has a rectangular waveform.
This waveform is usually equivalent to that obtained by integrating the waveform of a read signal in longitudinal magnetic recording. Thus, a disk drive adopting a perpendicular magnetic recording system needs a differentiating circuit to convert a read signal from the read head into a signal with a differentiated waveform in its signal processing circuit in order to decode (reproduce) recorded data from read signals. This signal processing circuit is called a data channel or a read/write channel, and it usually employs the PRML (Partial Response Maximum Likelihood) signal processing method.
In short, for practical use of a disk drive adopting a perpendicular magnetic recording system, a data channel used in a conventional longitudinal magnetic recording method can be reused by converting a rectangular waveform read signal into a differentiated waveform signal.
However, if a read signal from the read head is differentiated and equalized in a differentiating circuit, noise (system noise) contained in the read signal is emphasized in its high-frequency component. This makes it difficult to ensure a satisfactory signal-to-noise ratio for a drive system particularly with wide-range read signals (user data signals). In other words, this increases the error rate when reproducing recorded data from read signals.
There is a method of directly processing a rectangular waveform read signal in a perpendicular magnetic recording system without converting it into a differentiated waveform signal. This is PRML signal processing based on positive coefficient PR (Partial Response) equalization (corresponding to PR class 1 or 2), which is applicable to a perpendicular magnetic recording system. However, a rectangular waveform read signal contains a DC component, which causes distortion of the signal waveform and fluctuation in signal amplitude when the signal is transmitted through a circuit having a high low-frequency cut-off frequency.
It is to be noted that a signal read from a disk medium by the read head in a disk drive includes servo data signals recorded in a servo area, in addition to user data signals. The disk drive incorporates a servo system which controls the position of the magnetic head on the disk medium according to the servo data reproduced by the data channel. This servo system detects the head position depending on the amplitude of the servo signal, and the fluctuation in the amplitude of the servo signal must be suppressed.